Group III nitride semiconductors such as a gallium nitride (GaN) semiconductor (hereinafter referred to as group III nitride compound semiconductors or GaN type semiconductors, as the case may be) are attracting attention as the materials for blue light or ultraviolet light-emitting semiconductor elements. Blue laser diodes (LDs) are applied to high-density optical discs or high-density displays, and blue light-emitting diodes (LEDs) are applied to displays or illumination. Ultraviolet LDs are expected to be applied to biotechnology and the like, and ultraviolet LEDs are expected to be applied as ultraviolet light sources for fluorescent lamps.
Substrates composed of the group III nitride semiconductors (such as GaN) for use in LDs and LEDs are usually formed on sapphire substrates by heteroepitaxially growing group III nitride single crystals with vapor phase epitaxial growth methods. Examples of the vapor phase growth methods include the metal organic chemical vapor deposition method (MOCVD method), the hydride vapor phase epitaxy method (HVPE method) and the molecular beam epitaxy method (MBE method).
Instead of vapor phase epitaxial growth, methods for growing crystals in liquid phase have also been investigated. The nitrogen equilibrium vapor pressure at the melting point of the single crystal of group III nitride such as GaN or JUN is ten thousands atm or more. Accordingly, it has been accepted that the growth of GaN in the liquid phase requires the conditions set at 1200° C. (1473 K) and 8000 atm (8000×1.01325×105 Pa). In contrast, recently it has been shown that the use of an alkali metal such as Na as a flux enables the synthesis of GaN at a relatively low temperature of 750° C. (1023 K) and a relatively low pressure of 50 atm (50×1.01325×105 Pa).
Recently, in an ammonia-containing nitrogen gas atmosphere, a mixture composed of Ga and Na is melted at 800° C. (1073 K) and 50 atm (50×1.01325×105 Pa), and single crystals having a maximum crystal size of about 1.2 mm have been obtained by using the resulting melt, with a growth time of 96 hours (for example, see Patent Document 1).
There has also been reported a method in which a GaN crystal layer is formed as a film on a sapphire substrate by the metal organic chemical vapor deposition (MOCVD) method, and then a single crystal is grown by the liquid phase epitaxy (LPE) method (for example, see Patent Document 2).
Additionally, as the melt stirring method applied in the growth of the group III nitride crystal, various methods have been proposed, and for example, the following methods have been disclosed:
(a) A method in which a crystal is rotated (Patent Document 3)
(b) A method in which a melt-holding vessel is rotated or swung (Patent Documents 4 and 5)
(c) A method in which a melt is stirred by taking advantage of the temperature difference (Patent Document 6)
Patent Document 1: JP2002-293696A
Patent Document 2: JP2000-327495A
Patent Document 3: JP2002-68896A
Patent Document 4: JP2001-53013A
Patent Document 5: JP2005-247615A
Patent Document 6: JP2006-240959A